1. Field of the Invention
The present invention relates to an optical head servo device in an optical recording and reproducing apparatus. More specifically, the invention relates to a servo device which performs a servo operation to follow the track of an information recording disc and a temporal variation of a reproduction signal even when the light output power of an optical head is changed over in accordance with the operation modes such as reproduction, recording and erasing.
2. Description of Related Art
FIG. 1 shows a magneto-optical disc recording and reproducing apparatus of the type as is presently known.
In FIG. 1, a magneto-optical disc 1 has tracks separated by grooves, and is coated with a light-reflective magnetic film 2 in which signals are recorded magnetically. The disc 1 is rotated by a motor 3.
A linearly polarized light beam emitted from a semiconductor laser 4 is introduced, via a collimator lens 5, to a beam shaping prism 6, which shapes the elliptical light beam into a circular beam having an isotropic intensity distribution. The light beam output from the beam shaping prism 6 is passed through a beam splitter 7, reflected by a mirror 8, and focused by an objective lens 9 onto the magnetic film 2. While being reflected by the magnetic film 2, the light beam experiences a change (rotation) of its polarization plane in accordance with the signal that is recorded in the magnetic film 2 in the form of a variation of magnetization. The light beam reflected from the magnetic film 2 returns to the objective lens 9, and is introduced to a beam splitter 10 via the mirror 8 and beam splitter 7.
The light beam reflected by the beam splitter 10 is passed through a half-wave plate 11 and a convex lens 12, and input to a polarizing prism 13, where it is separated into P- and S-polarization light beams. The P-polarization light beam passed through the polarizing prism 13 enters a photodetector 14a. The S-polarization light reflected by the polarization prism 13 enters a photodetector 14b. A reproduction signal is obtained based on detection outputs of the photodetectors 14a and 14b.
The light beam transmitted through the beam splitter 10 is input to a 4-segmented photodetector 17 via a convex lens 15 and a cylindrical lens 16. Although a focus error signal and a tracking error signal are generated from a detection output of the 4-segmented photodetector 17, only the tracking error signal is described below. The detection output of the 4-segmented photodetector 17 is provided to a tracking error signal generation circuit 18, which generates a tracking error signal. The tracking error signal generation circuit 18 provides a tracking actuator 19 with a drive signal that is produced in accordance with the tracking error signal. Based on this signal, the tracking actuator 19 performs a fine adjustment of the position in the disc radial direction of the objective lens 9.
FIG. 2 is a block diagram showing a configuration of the tracking error signal generating circuit 18, in which the same parts as in FIG. 1 are represented by the same reference numerals.
In FIG. 2, among outputs of four photodetecting surfaces 17a-17d of the 4-segmented photodetector 17, the outputs of the photodetecting surfaces 17a and 17b are input to an adder 181 while the outputs of the photodetecting surfaces 17c and 17d are input to an adder 182. Respective outputs of the adders 181 and 182 are input to a subtracter 183, where their difference component is extracted as the tracking error signal. The one-dot-chain line drawn on the photodetector 17 in FIG. 2 corresponds to the track direction of the disc 1.
The tracking error signal thus generated is provided to a gain control circuit 184. The gain control circuit 184 amplifies, at a gain in accordance with a control signal, the tracking error signal to a predetermined level. An output signal of the gain control circuit 184 is provided to an equalizer 185. The control signal is to keep the open-loop gain of a tracking servo loop at an approximately constant level in accordance with a radius of the disc 1 that is known from address data recorded on the disc 1. After processed by the equalizer 185, the tracking error signal is provided, via a drive amplifier 18, to the tracking actuator 19 to drive it.
With the configuration as shown in FIGS. 1 and 2, the output power of the semiconductor laser 4 is changed over depending on whether the apparatus is in the reproduction period, recording period or erasing period. More specifically, the laser output power is controlled in the following manner. In the reproduction mode, the laser output power is kept constant, at level 1. During the recording mode, it is changed over between level 1 and level 2 that is higher than level 1 in accordance with an encoded signal to be recorded. During the erasing mode, it is kept at level 2.
While having an advantage that the laser output power is kept constant irrespective of a variation among the discs, this method is associated with a problem that the tracking error includes a transient component that occurs at the time of changing over the laser output power, which is illustrated in FIG. 3.
As shown in FIG. 3, if the laser output power is changed over during one rotation of the disc 1 with the switching of the operation mode in the order of a reproduction mode PB1, a recording/erasing mode REC1, a reproduction mode PB2 and a recording/erasing mode REC2, the tracking error signal takes a waveform as shown in part (a) of FIG. 3. It can be seen that a large-level error signal occurs every time the operation mode is changed over. This will cause an unstable tracking servo operation in the case where the track pitch is smaller than the laser spot diameter.
This problem, which occurs at the time of changing over the laser output power, results from a phenomenon that a wavelength change of the light beam in association with the changing over of the laser output power causes a change of the refractive index of the beam shaping prism 6 that has the function of shaping the elliptical light beam emitted from the semiconductor laser 4 into the circular beam. This refractive index change causes a deviation of the beam incident position on the disc 1 in both of the tracking control direction and the tangential direction, which produces instantaneous, large tracking and tangential errors.